1. Technical Field
The present invention relates to a transmission circuit using a polar modulation method which is used in a wireless communication system such as a mobile phone, a wireless LAN, and the like, and more particularly, to a transmission circuit which adjusts a difference between a signal delay amount in an amplitude path and a signal delay amount in a phase path in a polar modulation method and a communication apparatus using the transmission circuit.
2. Background Art
Recently, as a transmission circuit which operates highly efficiently, there has been an increasing interest in a transmission circuit using a polar modulation method. In the polar modulation method, an input signal is separated into a phase signal representing a phase component and an amplitude signal representing an amplitude component, and processed. The separated phase signal is multiplied by an oscillation signal generated by an oscillator to obtain a phase signal having a constant amplitude, that is, a phase modulation signal. The phase modulation signal is synthesized with the separated amplitude signal by using a saturation amplifier and outputted as a transmission signal.
In the polar modulation method, a phase signal and an amplitude signal are separately processed. Accordingly, when a phase modulation signal is synthesized with an amplitude signal, the saturation amplifier can be operated in a saturation region, thereby allowing generation of a transmission signal with high power efficiency.
At the same time, in the polar modulation method, because the phase signal and the amplitude signal are separately processed, if there is a difference between an amount of signal delay in an amplitude path (a path in which the separated amplitude signal is processed) and an amount of signal delay in a phase path (a path in which the separated phase signal is processed), the spectrum of the transmission signal obtained by synthesizing the phase modulation signal with the amplitude signal spreads, resulting in deterioration of adjacent channel interference characteristics. Consequently, in the polar modulation method, a difference between a signal delay amount in the amplitude path and a signal delay amount in the phase path needs to be adjusted.
Therefore, a conventional transmission circuit using the polar modulation method outputs a transmission signal and returns a part of the transmission signal as a feedback signal. The conventional transmission circuit using the polar modulation method calculates a difference between a signal delay amount in the amplitude path and a signal delay amount in the phase path based on the input signal and the feedback signal, and adjusts the difference (for example, see Patent Literature 1 and Patent Literature 2).
FIG. 6 shows a configuration example of a transmission circuit using a conventional polar modulation method, which adjusts a difference between signal delay amounts by using feedback control. In FIG. 6, a conventional transmission circuit 100 includes a signal generation section 101, a delay adjustment section 102, a regulator 103, a phase modulation section 104, an amplitude modulation section 105, and a coupler 106.
The signal generation section 101 processes an input signal and thereby generates an amplitude signal R and a phase signal θ. Based on the amplitude signal R and the phase signal θ generated by the signal generation section 101, and a feedback signal FB fed back from the coupler 106, the delay adjustment section 102 calculates a difference between a signal delay amount in the amplitude path and a signal delay amount in the phase path. Then, the delay adjustment section 102 controls an output timing of the amplitude signal R or the phase signal θ so that the difference between the signal delay amounts is eliminated. An amplitude signal R′ and a phase signal θ′ outputted from the delay adjustment section 102 are inputted to the regulator 103 and the phase modulation section 104, respectively. The regulator 103 provides the amplitude modulation section 105 with a voltage control signal in accordance with the inputted amplitude signal R′. By using the inputted phase signal θ′, the phase modulation section 104 modulates an oscillation signal generated by an oscillator (not shown) to generate a phase modulation signal. The amplitude modulation section 105 is typically a power amplifier (PA), and amplitude-modulates the phase modulation signal outputted from the phase modulation section 104 by the voltage control signal provided by the regulator 103 to generate a modulation signal and outputs the modulation signal as a transmission signal. The coupler 106 divides the transmission signal into two signals, which are an output signal and a feedback signal. The output signal is transmitted in the air via an antenna (not shown). Further, the feedback signal FB is inputted to the delay adjustment section 102 and used for calculating a difference between a signal delay in the amplitude path and a signal delay in the phase path.
With the above configuration, in the conventional transmission circuit 100, a difference between a signal delay amount in the amplitude path and a signal delay amount in the phase path can be adjusted. However, in the conventional transmission circuit 100, a problem remains that a power loss caused by the coupler 106 is large, resulting in decrease in power efficiency of the entire circuit.
In order to solve the problem, there has been proposed a technology to adjust a difference between signal delay amounts without using a coupler (for example, see Patent Literature 3). FIG. 7 shows a configuration example of a transmission circuit using a conventional polar modulation method without a coupler. In FIG. 7, a conventional transmission circuit 200 includes the signal generation section 101, a delay adjustment section 202, the regulator 103, the phase modulation section 104, and the amplitude modulation section 105.
The signal generation section 101 processes an input signal, and thereby generates an amplitude signal R and a phase signal θ. The amplitude signal R is inputted to the regulator 103 and the delay adjustment section 202, and the phase signal 0 is inputted to the delay adjustment section 202. The regulator 103 provides the amplitude modulation section 105 with a voltage control signal in accordance with the inputted amplitude signal R. The delay adjustment section 202 receives the amplitude signal R and the phase signal θ generated by the signal generation section 101, and the voltage control signal provided by the regulator 103. Then, the delay adjustment section 202 obtains an amount of signal delay in the amplitude path based on the amplitude signal R and the voltage control signal, and controls an output timing of the phase signal θ based on the amount of signal delay. By using the adjusted phase signal θ′ outputted from the delay adjustment section 202, the phase modulation section 104 modulates an oscillation signal generated by an oscillator (not shown) to generate a phase modulation signal. The amplitude modulation section 105 amplitude-modulates the phase modulation signal outputted from the phase modulation section 104 by the voltage control signal provided by the regulator 103 to generate a modulation signal and outputs the modulation signal as a transmission signal.
With the above configuration, in the conventional transmission circuit 200, a difference between a signal delay amount in the amplitude path and a signal delay amount in the phase path can be adjusted without power loss by a coupler